In electrophotography apparatuses where an electrostatic latent image is formed on a photoconductive medium; a toner is caused to be deposited on the charged image area, and the powder image thus formed is then transferred to paper and fused there by heat, a dry type involving a powder toner has been widely used.
The powder toner used in the dry type electrophotography apparatus, however, tends to be easily carried about in the air, and has relatively poor resolution because of its large particle sizes ranging from 7.about.10 .mu.m.
Where a high resolution is needed, therefore, a wet type electrophotography using a liquid toner is preferred. The liquid toner has particle sizes as small as approx. 1 .mu.m and a high charging capacity, with the result that the toner image is less susceptible to turbulence, leading to improved resolution.
In conventional wet type electrophotography apparatuses, a low-viscosity liquid toner composed of 1.about.2% of toner mixed in an organic solvent is used as a developer. This type of developer solution, however, arouses environmental concerns since it uses an organic solvent that causes hazards to human bodies, and it is generally consumed in a large amount due to its low toner concentration.
It is against this background that an invention involving a wet type lectrophotography apparatus using a high-viscosity, high-concentration developer solution consisting of a high-concentration toner dispersed in silicone oil was disclosed in international Disclosure Number "WO95/08792".
The use of this liquid toner is beneficial because it is not hazardous to human bodies, and because the toner of a high concentration eliminates the need for a large amount of developer solution.
A known method of fixing a toner deposited on an electrostatic latent image on the photoconductive medium is to transfer the toner particles deposited on the photoconductive medium directly onto a printing medium without using an intermediate transfer medium. In processing color images, however, the method of transferring the toner deposited on the photoconductive medium directly onto a printing medium relying on the force of electric field, and fusing the toner there by heating the printing medium would have to pass the printing medium over the photoconductive medium at least three times (four times when realizing black color with a single toner). This would limit the type and material of printing medium, posing a practical difficulty.
Thus, when processing color images, a method of transferring the toner deposited on the photoconductive medium onto an intermediate transfer medium, and fusing the toner on the printing medium by heating the intermediate transfer medium has been adopted. That is, a method of electrostatically transferring the toner particles deposited on the photoconductive medium onto an intermediate transfer medium, and fusing the toner particles consisting of a thermoplastic resin, a pigment, etc. into a viscous molten state by heating the intermediate transfer medium and transferring the toner particles onto the printing medium by forcing them onto the printing medium is known. This method is being widely used especially in processing color images since it has advantages that a toner image can be transferred stably without relying on electrical properties, such as resistance values, of the printing medium, and that the printing medium is passed over the intermediate transfer medium only once.
When implementing this method of fixing the toner on the printing medium, a construction as shown in FIG. 16 has been adopted in which an intermediate transfer roll made of a hollow metallic drum is provided as an intermediate transfer medium 15, with a halogen heater installed inside the hollow drum to heat the entire intermediate transfer medium.
With this construction, however, the heat of the intermediate transfer medium 15 whose surface temperature is kept high at all times is readily transmitted to the photoconductive medium 10, adversely affecting the performance of the photoconductive medium 10. If a high-viscosity, high-concentration developer solution consisting of a high-concentration toner dispersed in silicone oil is used, the intermediate transfer medium can be heated in a manner different from the conventional manner without giving adverse thermal effects on the photoconductive medium since the heat capacity of the toner can be reduced due to small toner particles.
In this transfer method, temperature settings for the intermediate transfer medium, the toner, the printing medium and the pressure roller are essential. The temperatures of toner particles and the printing medium at the time when the toner particles come in contact with the printing medium for transfer should preferably be higher than the softening point of the toner particles, so that the toner particles and the printing medium can be brought into close contact by a backup pressure from the rear surface of the printing medium, with the result that transfer is achieved by the adhesion of the fused toner particles or the toner layer. To achieve the aimed transfer efficiency of 100%, it is necessary to heat and fuse the toner particles, which form an image on the intermediate transfer medium, into an integrated film layer, and maintain the adhesion between the surface of the intermediate transfer medium and the fused toner layer at a sufficiently lower level than the cohesive force in the fused toner layer and the adhesion between the fused toner layer and the printing medium.
Moreover, there can be two toner heating methods; the one of heating the entire intermediate transfer medium at all times, and the other of heating the intermediate transfer medium by making contact with the surface thereof as necessary. In both cases, an efficient thermal transfer of a toner image is required, taking into consideration power consumption, ready-to-print capability, changes in the state of toners when different colors are deposited on the intermediate transfer medium, and thermal effects on the photoconductive medium and other processing materials.